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See-through teeth

THEY won’t make a trip to the dentist painless, but researchers at Toshiba’s
laboratory in Cambridge hope to make it safer by doing away with X-rays. They
have created a three-dimensional image of an extracted tooth and hope to use
their technique on patients in the dentist’s chair.

Their new approach is called terahertz pulse imaging (TPI) and relies on
terahertz radiation—also known as millimetre waves, which lie between the
infrared and microwave frequencies on the electromagnetic spectrum. Many common
materials are semi-transparent to radiation at these frequencies, so there are
many possible applications in medicine, industry, or even military intelligence.
For example, the Toshiba team is looking at quality control for food and
semiconductors, and security applications such as detecting concealed
explosives. And the military is keen to develop cameras that can see through
walls.

Unlike X-rays, terahertz radiation is non-ionising—it is not energetic
enough to knock electrons out of their shells, so it should be safer for medical
imaging. Also, the power levels needed to build an image are lower than the
background terahertz radiation we encounter every day, says project leader Don
Arnone.

But because it was so hard to produce emissions at these frequencies,
terahertz radiation has not been used much. Previous work has concentrated on
producing terahertz waves by combining two approaches. Researchers tend to
generate lower, microwave frequencies using accelerated electrons, while
emissions in the higher-frequency infrared and visible regions are produced
using lasers.

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“We came to the conclusion that it was too difficult to combine the two
techniques, so we cheated,” says Arnone. His team combined off-the-shelf laser
technology with Toshiba’s expertise in semiconductors. They bombarded a
specially engineered semiconductor with rapid pulses of laser light, triggering
brief pulses of terahertz waves.

The team used a technique called “time-of-flight” to create 3D images of
internal structures. By measuring the time delay of a pulse of radiation as it
passes through an object and is reflected off its internal surfaces, TPI can
reveal the precise distances to these surfaces. So by scanning a beam across it,
a computer can map the object’s internal structure and display the digitised
image. TPI can also distinguish between different tissues by the absorption
“fingerprints” they produce. Toshiba can already distinguish between muscle, fat
and kidney tissue in samples of pork. Arnone hopes that the technique may one
day be able to identify diseased tissue such as tumours.

“It looks like an interesting development,” says Jim Elliott, a dental
biophysicist at Queen Mary and Westfield College in London. “But there will need
to be a lot of work before it will be useful.” The crucial question is whether
any device can be made sensitive enough. Before you could decide on treatment,
he says the device would need to tell you whether the decay had reached the
dentine or was merely affecting the tooth enamel.